This invention relates to robots for performing surgical tasks.
Various designs of robot have been proposed for performing or assisting in surgery. However, many robot designs suffer from problems that make them unsuitable for performing a wide range of surgical procedures. A common reason for this is that in order for a surgical robot to work well in a wide range of surgical situations it must successfully balance a set of demands that are particular to the surgical environment.
Normally a surgical robot has a robot arm, with a surgical instrument attached to the distal end of the robot arm.
A common demand on a surgical robot is that its robot arm should offer sufficient mechanical flexibility to be able to position the surgical instrument in a wide range of locations and orientations so that the working tip of the surgical instrument (the end effector) can reach a range of desired surgical sites. This demand alone could easily be met by a conventional fully flexible robot arm with six degrees of freedom, as illustrated in FIG. 1. However, secondly, a surgical robot must also be capable of positioning its arm such that the end effector of the instrument is positioned very accurately without the robot being excessively large or heavy. This requirement arises because unlike the large-scale robots that are used for many other tasks, (a) surgical robots need to work safely in close proximity to humans: not just the patient, but typically also surgical staff such as anaesthetists and surgical assistants, and (b) in order to perform many laparoscopic procedures it is necessary to bring multiple end effectors together in close proximity, so it is desirable for surgical robot arms to be small enough that they can fit closely together. Another problem with the robot of FIG. 1 is that in some surgical environments there is not sufficient space to be able to locate the base of the robot in a convenient location near the operating site.
Many robots have a wrist (i.e. the terminal articulated structure of the arm) which comprises two joints that permit rotation about an axis generally along the arm (“roll joints”) and between them one joint that permits rotation about an axis generally transverse to the arm (a “pitch joint”). Such a wrist is shown in FIG. 2, where the roll joints are indicated as 1 and 3 and the pitch joint is indicated as 2. With the wrist in the configuration shown in FIG. 2 the axes of the joints 1 to 3 are indicated as 4 to 6 respectively. This wrist gives an instrument 7 the freedom of movement to occupy a hemisphere whose base is centred on axis 4. However, this wrist is not well suited for use in a surgical robot. One reason for this is that when the pitch joint 2 is offset by just a small angle from the straight position shown in FIG. 2 a large rotation of joint 1 is needed to produce some relatively small lateral movements of the tip of the instrument. In this condition, when the pitch joint is almost straight, in order to move the end effector smoothly in a reasonable period of time the drive to joint 1 must be capable of very fast operation. This requirement is not readily compatible with making the arm small and lightweight because it calls for a relatively large drive motor and a sufficiently stiff arm that the motor can react against it without jolting the position of the arm.
Another common demand on a surgical robot is that it should be designed such that forces which are applied to the surgical instrument are measurable. Because the surgeon is not directly in contact with the surgical instrument during robotic surgery, tactile feedback is lost compared to manual surgery. This lack of tactile sensation means that the surgeon does not know how much force is being applied when using the surgical instrument. This affects the surgeon's dexterity. Additionally, too much exerted force can cause internal damage to the patient at the surgical site, and can also damage the surgical instrument and the robot arm. By measuring the forces applied to the surgical instrument, these can be implemented in a force feedback mechanism to provide force feedback to the surgeon. For example, haptic technology can be used to convert the measured forces into physical sensations in the input devices that the surgeon interacts with, thereby providing a replacement for the tactile sensation of manual surgery.
The wrist shown in FIG. 2 enables some force measurement. Specifically, forces which are exerted vertically on the surgical instrument (i.e. perpendicular to axis 5) illustrated by arrow 8, can be measured using a torque sensor applied to the pitch joint 2. However, forces which are applied to the surgical instrument from other directions cannot be sensed using torque sensors applied to the wrist. Other techniques are possible to detect forces applied from other directions, for example by using strain gauges. However, these other techniques require more sensors to be applied to the robot, and hence more electronics are required to route and process the data, which increases the weight and power requirements of the robot.